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Abstract:

The invention relates to a Fresnel solar collector arrangement consisting
essentially of a receiver (1) and a mirror arrangement associated with
the receiver (1). The arrangement is temperature-compensated by the use
of materials with the same temperature expansion coefficient for the
receiver mast (2) and the mirror supporting framework (4), and the
adjustment of the primary mirror (6, 6') in relation to the sun by a
mechanical coupling of the mirror is simplified by means of an
electromotive connecting rod.

Claims:

1. Fresnel solar collector arrangement having at least one receiver (1)
mounted on a receiver supporting framework, elevated relative to several
primary mirrors (6, 6') that are disposed on a mirror supporting
framework (4) on both sides of the receiver (1) so as to pivot, in such a
manner that the solar radiation reflected by the primary mirrors (6) is
focused at least essentially onto the receiver (1), and the primary
mirrors (6) are, in each instance, made to track the sun, wherein the
mirror supporting framework (4) that extends bilaterally outward from the
receiver supporting framework is mounted in a fixed location in the
region of the receiver supporting framework and/or in connection with the
receiver supporting framework and, for the remainder is mounted to slide,
i.e. in constraint-free manner, at least to a great extent.

2. Fresnel solar collector arrangement according to claim 1, wherein the
receiver supporting framework and the mirror supporting framework (4) are
made from a material having at least largely identical material expansion
coefficients, preferably from steel 37, in each instance.

3. Fresnel solar collector arrangement according to claim 1, wherein the
receiver (1) comprises an absorber tube that is mounted in an elevated
manner by means of a row of receiver masts (2), preferably disposed in an
imaginary straight line, which essentially form the receiver supporting
framework, and that the mirror supporting framework (4) comprises
supporting rails (5) that are spaced apart from one another and connected
in a framework-like manner by means of appropriate struts, if necessary,
whereby these supporting rails (5) extend outward at least essentially
orthogonally from an imaginary straight line of the receiver masts (2)
disposed in a row, and that these supporting rails (5) are fixed in place
in connection with the receiver masts (2), or in the foundation region of
the receiver masts (2), and otherwise are mounted to slide.

4. Fresnel solar collector arrangement according to claim 1, wherein the
primary mirrors (6, 6') disposed on a common supporting rail (5) along an
imaginary line parallel or perpendicular to the longitudinal expanse of
the absorber tube of the receiver (1), are combined to form a primary
mirror group, whereby the primary mirrors (6, 6') of the primary mirror
group are mechanically coupled by means of a common setting element and,
as a result, are made to track the sun together.

5. Fresnel solar collector arrangement according to claim 4, wherein the
primary mirrors (6) assigned to one or more supporting rails (5) disposed
in an imaginary extension relative to one another are combined to form a
primary mirror group.

6. Fresnel solar collector arrangement according to claim 5, wherein the
primary mirrors (6, 6') of the primary mirror group are, in each
instance, mounted to pivot on the supporting rail(s) (5), whereby these
primary mirrors (6) are rigidly connected with one another by means of a
connecting rod (10, 10'), preferably driven by an electric motor, and can
be pivoted together, relative to the absorber tube of the receiver (1),
by means of linear movement of the connecting rod (10, 10') in the
direction of the longitudinal expanse of the connecting rod (10, 10'), in
terms of their respective angle of incidence.

7. Fresnel solar collector arrangement according to claim 6, wherein the
primary mirrors (6, 6') of a primary mirror group are firmly connected
with one another by means of a tracking shaft (17), for the purpose of
common pivoting, which shaft is essentially disposed parallel to the
longitudinal expanse of the absorber tube, to rotate relative to the
supporting rails (5).

8. Fresnel solar collector arrangement according to claim 7, wherein the
tracking shaft (17) is mounted in a roller bearing block (15) that
surrounds the shaft, whereby the shaft is held in the roller bearing
block (15) by means of a multiplicity, preferably three, of barrel-shaped
roller elements (16, 16', 16''), in such a manner that axial rotation of
the tracking shaft (17) is made possible.

9. Fresnel solar collector arrangement according to claim 5, wherein the
connecting rod (10, 10') can be displaced by a motor in the direction of
its longitudinal expanse, by means of a linear motor (11).

10. Fresnel solar collector arrangement according to claim 6, wherein the
linear motor (11) is preferably disposed in the region of the receiver
mast (2), and one or more connecting rods (10, 10') are driven by the
linear motor (11), whereby the primary mirrors (6) disposed on the right
of the receiver (1) are moved opposite the primary mirrors (6') on the
left of the receiver (1), for the purpose of tracking, whereby this
opposite movement is implemented by means of a corresponding deflection
mechanism provided on only one side of the receiver (1).

12. Fresnel solar collector arrangement according to claim 10, wherein the
receiver (1) additionally comprises a secondary reflector assigned to the
absorber tube, which essentially absorbs the scattered radiation
reflected by the primary mirrors (6, 6'), and deflects it onto the
absorber tube.

13. Fresnel solar collector arrangement according to claim 1, wherein the
dimensions and spacings of the primary mirrors (6, 6') are dimensioned in
such a way that mutual shadowing of the primary mirrors (6, 6') is
precluded, at least to a great extent.

Description:

[0001]The invention relates to a Fresnel solar collector arrangement.

[0002]This is understood to mean a line-focusing system in which multiple
mirror strips disposed parallel to a receiver are made to track the
position of the sun, and the solar radiation is guided onto a fixed
absorber tube in which a heat storage medium flows. In addition, a
secondary reflector assigned to the absorber tube guides the radiation
onto the focal line essentially formed by the absorber tube. The absorber
tube and the secondary reflector form the receiver disposed in elevated
manner above the mirror strips. Such a Fresnel solar collector is
currently in operation in Australia, for example, in a field trial. The
heat that is produced can be utilized as process heat, or it can be
converted into an electric current, for example by means of a Stirling
motor.

[0003]The advantage of Fresnel solar collectors as compared with
conventional parabolic trough collectors lies in their significantly
simplified structure. Parabolic trough collectors consist of a reflector
that has the shape of a parabolic cylinder. Here also, the light is
focused onto a line, the focal line. The absorber tube of the parabolic
trough collector, which absorbs the concentrated radiation and passes it
on to the medium flowing through, is situated in this line. In this
connection, the medium is typically heated to values of approximately
400° C. In order to improve the efficiency, the absorber can be
surrounded by a glass tube. A vacuum prevails in the interstice between
absorber tube and glass tube, for insulation. The "solar steam" produced
in this way can also be utilized directly for process heat applications,
or for conventional steam power plants and cogeneration power plants.

[0004]Alternatively, flat collectors and CPC collectors are known as
further types of collectors.

[0005]In this connection, the efficiency of the Fresnel solar collector
essentially depends on how well the reflected solar radiation is focused
onto the absorber tube. For this purpose, it is practical to make the
primary mirrors assigned to the absorber tube track the sun. Only in this
way can acceptable efficiencies be achieved for the system. This usually
takes place by means of an electric motor assigned to each primary
mirror. In turn, the electric motors are usually provided with a timing
device, so that tracking is more a question of controlling than of
regulating.

[0006]A significant problem with Fresnel solar collectors is that such
arrangements achieve their best efficiencies in regions with the highest
incoming solar radiation, for example in desert regions, where extreme
temperature variations from degrees below zero to degrees above zero of
far in excess of 40° C. are at least not unusual. The materials
and supporting structures used are exposed to considerable stresses, in
this connection, whereby thermal deformations of the material are
practically unavoidable and therefore can lead to angular deviations
within the entire system, which can be manifested in the double-digit
percentage range in the efficiency of the entire system. Even a small
angle deviation in the supporting structure of the mirror arrangement can
lead to having a large part of the radiation reflected by the primary
mirrors not focused onto the absorber tube but rather reflected past the
absorber tube. Furthermore, individual control, i.e. individual
regulation and coordination of the various electric motors for tracking,
i.e. controlling the panning movement of the mirrors is accompanied by
considerable regulation and control effort, making the system somewhat
susceptible to malfunctioning.

[0007]Proceeding from this prior art, the invention is based on the task
of configuring the system more robustly, overall, and of improving its
efficiency as much as possible.

[0008]This task is accomplished by means of a Fresnel solar collector
arrangement according to the main claim. Advantageous embodiments can be
seen in dependent claims 2 through 11.

[0009]Because, according to the main claim, the mirror supporting
framework is mounted in a fixed position in the region of the receiver
supporting framework and/or in connection with the receiver supporting
framework, and furthermore is mounted in sliding manner, i.e. free from
constraint, it is assured, in the case of unavoidable thermal expansion
of the supporting framework as a result of the effects of heat, that the
mirror supporting framework balances out these corresponding changes.

[0010]This succeeds even better if the receiver supporting framework and
the mirror supporting framework are essentially made from the same
material and are essentially mounted in fixed manner at the same
location. If thermal expansions or contractions of the material occur,
one can at least approximately assume that the alternating expansions of
the supporting frameworks take place to the same extent. For example, the
receiver mast is then expanded, as the result of the effect of heat, in
approximately the same way as the mounting rails of the primary mirrors
disposed as the mirror framework. Because the receiver framework and the
mirror supporting framework are at least essentially disposed orthogonal
relative to one another and are made from the same material, and
therefore have the same expansion coefficient, it is assured that the
angle relationships do not change relative to one another or, at most,
change only slightly. However, this is possible only if both the receiver
framework and the mirror supporting framework are mounted in
constraint-free manner, i.e. only one of at least two required supports
is fixed in place. This surprisingly simple solution eliminates
complicated re-adjustments for material expansions or contractions, or
making a largely hopeless attempt to use materials that are more or less
temperature-independent. The use of such materials is usually ruled out
for cost reasons alone.

[0011]In a concrete embodiment, the receiver of the Fresnel solar
collector arrangement can be mounted as an absorber tube on a row of
receiver masts, whereby the mirror supporting framework can also be
mounted in a fixed location at the same point, if necessary using the
same concrete pedestal. In this connection, receiver mast and mirror
supporting framework are advantageously made from steel 37, in each
instance, and therefore exhibit largely the same expansion coefficient.

[0012]In an advantageous further development, some of the primary mirrors
mounted on the mirror supporting framework are combined to form a primary
mirror group, which in turn are mechanically coupled by means of a common
mechanical setting element, for tracking purposes, and thus are made to
track the sun. Because of the use of a common setting element,
complicated coordination, complicated control and regulation of the
electric motors used is eliminated, at least within the primary mirror
group in question. Instead, the entire primary mirror group can be
adjusted by means of a common setting element, whereby the relative angle
relationship between the primary mirrors is maintained at all times.
Again, this is based on the actually trivial recognition of the law
governing radiation, that the relative angle adjustments of the primary
mirrors required in the course of tracking the sun, where these mirrors
are disposed one behind the other, in an imaginary orthogonal line
relative to the absorber tube, which is disposed at a distance and
elevated, are the same relative to one another. Incidentally, this would
also apply to the primary mirrors disposed on an imaginary line parallel
to the absorber tube.

[0013]In this connection, the embodiment of the collector arrangement
explained above, with mechanical coupling for the common panning movement
of the primary mirrors by means of a common setting element, is also
advantageous independent of the constraint-free mounting of the mirror
supporting framework.

[0014]This common panning movement is achieved as a result of connecting
the primary mirrors of a primary mirror group by means of a tracking
shaft. Due to the movement of the connecting rod when aligning the
primary mirrors, a rotation of the tracking shaft is brought about, which
is uniformly transferred to the entire primary mirror group by means of
the connection.

[0015]It is advantageous if the tracking shaft is mounted, at regular
intervals, in roller bearing blocks that surround the shaft but that only
support it using roller elements. These roller elements permit axial
rotation of the tracking shaft, and are formed in barrel-like shape, in
other words are essentially cylindrical, whereby their mantle surfaces
bulge out. This shape makes it possible to dispose the tracking shaft not
only along planar surfaces but also, if required, to guide it along its
path over different heights. The shaft can be positioned at a slant on
the roller elements, so that simultaneous slanted positioning of the
roller bearing block can be eliminated.

[0016]In a concrete embodiment, mechanical coupling of the primary mirrors
combined to form a group can be implemented by means of a common
connecting rod, by way of which the primary mirrors mounted on the mirror
supporting framework so as to pivot are pivoted relative to the absorber
tube, as a function of the position of the sun, i.e. the time of day or,
to say it better, they are made to track the sun.

[0017]In an advantageous embodiment, the connecting rod is driven by an
electric motor, using a linear motor, whereby the connecting rod, which
is disposed orthogonally relative to the longitudinal expanse of the
absorber tube, is moved inward or outward, as a function of the sun's
position, by means of the linear motor.

[0018]In an advantageous embodiment, water vapor or thermal oil flows
inside the absorber tube and is heated to a temperature of up to
approximately 400° C. by the reflected radiation. The thermal
medium heated in this way can then be passed to further use, in known
manner, or can be used to produce electricity.

[0019]In order to further improve the efficiency of the arrangement
despite the improved angular accuracy of the arrangement, a secondary
reflector is additionally assigned to the absorber tube, which reflector
surrounds the absorber tube essentially like a shield, and thus captures
and deflects possible scattered radiation from the primary mirrors, in
such a way that this scattered radiation is also focused onto the
absorber tube.

[0020]Thus the secondary reflector is also disposed so that the absorber
tube lies essentially in the focal line of the secondary reflector.

[0021]In a further embodiment of the electric motor drive, the linear
motor is also disposed essentially centrally, i.e. approximately in the
region of the imaginary line formed by the receiver masts disposed in a
row. In this connection, when one and the same linear motor is used for
turning, one or more primary mirror groups driven by one or more
connecting rods, on the left of the absorber tube, and one or more
primary mirror groups driven by one or more connecting rods, on the right
of the absorber tube, can be driven in such a way that a time-controlled
panning movement of the primary mirrors, i.e. a panning movement that
tracks the sun, takes place relative to the absorber tube.

[0022]The necessarily opposite movement of the primary mirrors on the
right of the absorber tube in comparison to the primary mirrors on the
left of the absorber tube is implemented by means of a deflection
mechanism for the linear movement of the connecting rod, assigned to only
one of the two sides.

[0023]In an advantageous embodiment, the linear motors can be connected to
a common control and/or regulation unit, since the relative movements to
be carried out by the connecting rods are exactly identical over the
entire length of the absorber tube, and thus common regulation is
possible for the entire system.

[0024]The invention will be explained in greater detail below, using an
exemplary embodiment only shown schematically in the drawings.

[0025]The drawing shows:

[0026]FIG. 1: a Fresnel solar collector arrangement in cross-section,

[0027]FIG. 2: a detail of the Fresnel solar collector arrangement in a
schematic diagram, and

[0028]FIG. 3: a control diagram for the Fresnel solar collector
arrangements shown in FIGS. 1 and 2.

[0029]According to the illustration in FIG. 1, the Fresnel solar collector
arrangement consists of a receiver 1 mounted on a receiver mast 2. For
this purpose, the receiver mast 2 is mounted in a fixed bearing 3 that
simultaneously represents the center axis of a mirror supporting
framework 4 disposed with angle symmetry. In this connection, the mirror
supporting framework 4 essentially consists of supporting rails 5 made
from the same material as the receiver mast 2, namely steel 37 in the
case of the present exemplary embodiment, and extend orthogonally
outward, in each instance, from the longitudinal axis of the receiver 1.
In this connection, the receiver 1 essentially consists of an absorber
tube in which a thermal medium that acts as a heat storing material
flows. This can be simple steam or a thermal oil. The absorber tube is
generally surrounded by a secondary reflector that captures any stray
radiation of the mirror arrangement and deflects it onto the absorber
tube. The primary mirrors 6, 6' are mounted to pivot on both sides of the
supporting structure, i.e. essentially with mirror symmetry, on mirror
paths relative to the receiver 1 set up in elevated manner. In this
connection, the mirror paths are mounted on the mirror supporting
framework 4 essentially in such a way that the solar radiation acting on
the Fresnel solar collector arrangement is reflected and deflected in
such a way that it is focused onto the absorber tube in the region of the
receiver 1. Ideally, the absorber tube forms the focal line of the
primary mirrors 6, 6' mounted on the mirror supporting framework 4. In
this connection, several primary mirrors 6, 6' are to each receiver 1 at
a different distance, i.e. at an increasing orthogonal distance from the
central axis of the mirror supporting framework 4 defined by the absorber
tube.

[0030]Relative to the setup base, the mirror supporting framework 4 itself
is, in turn, mounted with foot elements 7 connected only by means of
slide bearings to the supporting rails 5, which extend in fixed manner,
orthogonal relative to the longitudinal expanse of the receiver 1. Thus,
in concrete terms, the receiver mast 2 and the supporting rails 5, which
are disposed one behind the other in the longitudinal expanse of the
receiver 1, are fixed in place only in the fixed bearing 3, and otherwise
are mounted in constraint-free manner, so as to slide. Since both the
receiver mast 2 and the supporting rails 5 are made from steel 37 and
therefore possess essentially identical expansion coefficients, any
thermal expansion of the two supporting frameworks is also essentially
the same. The longitudinal expansion of the receiver mast 2 is thus
essentially compensated in that any angle error in the arrangement, with
the possible consequence that the absorber tube moves out of the focal
line of the mirror arrangement, is compensated by a similar expansion of
the supporting rail 5.

[0031]The Fresnel solar collector arrangement according to FIG. 1 is thus
essentially temperature-compensated in self-regulating manner, in that
any material expansions and contractions resulting from the absolutely
normal extreme temperature variations in the regions of use of Fresnel
solar collector arrangements are reciprocally balanced out. As a result,
the losses due to scattering of the reflected radiation, which have a
very negative effect on the yield factor of the system, are avoided to a
great extent. Complicated techniques for compensating the changes in
length of the materials used, due to temperature, can therefore be
eliminated, to a great extent.

[0032]According to FIG. 2, the arrangement is advantageously supplemented
in that the primary mirrors 6, 6' assigned to the individual supporting
rails 5, are, in each instance, connected with the supporting rail 5, in
each instance, by means of a mirror support 8, 8', so as to pivot. In
this connection, it is known from the prior art to assign a separate
electric motor to each primary mirror 6, 6' and to achieve tracking of
the primary mirrors 6, 6' according to the position of the sun relative
to the receiver 1, using this electric motor drive. According to FIG. 2,
several primary mirrors 6, 6' are combined to form a primary mirror group
that is characterized by being mechanically coupled with one another by
means of a common setting element, namely a connecting rod 10, 10'. The
connecting rod 10, 10' is driven in linearly displaceable manner, by an
electric motor, using a linear drive 11, whereby the movement of the
connecting rods 10, 10' on the left and right of the receiver mast 2, and
thus the movement of the receiver 1, go in opposite directions by means
of a deflection mechanism not illustrated further here. The connecting
rods 10, 10' on the left and right of the receiver 1 are thus either both
moved inward or both moved outward. This is understood to mean that one
of the two connecting rods 10 or 10' acts only indirectly on the primary
mirrors 6, 6', namely by way of a deflection mechanism that leads to the
aforementioned opposite movement. This, in turn, brings about the result
that the mirrors disposed on the right and left are turned toward or away
from the centrally disposed reflector or absorber tube at precisely the
same angle relationship. The solution shown according to FIG. 2 therefore
makes it possible to create mechanical coupling by way of a simple
connecting rod 10, 10', using a single electric motor, and thus to
eliminate complicated coordination of several individual electric motors,
at least along one supporting rail 5, i.e. within a primary mirror group,
and, instead, to allow precise tracking following the position of the
sun, using a single common linear drive, because of the angle accuracy of
the arrangement.

[0033]In this connection, according to the schematic diagram in FIG. 3,
this can be a control unit and/or a regulation unit. According to the
illustration in FIG. 3, a common regulator 12 is assigned to the linear
motors 11, 11', 11'', to which one or more connecting rods 10, 10' or
supporting rails 5 are assigned, in each instance. In the simplest case,
this regulator 12 can be controlled in time-controlled manner, in the
sense of a control, according to a predefined program that associates
every time of day with a certain position of the sun and thus with an
angle position of the primary mirrors 6. For this purpose, the regulator
12 is data-connected to a time detection device 14. Alternatively,
however, the regulator 12 can also be connected to a true actual
value/reference value comparator 13, whereby the actual value and target
value either compare the real position of the sun to the target value
default or, on the other hand, the efficiency of the system is directly
fed back to the regulating variable, for example by evaluating the
radiation intensity achieved or the current yield of electricity as the
actual value, in order to determine any regulatory deviation. The angle
positioning of the primary mirrors 6, 6' can then be re-adjusted using
the setting element. Thus, understood correctly, the connecting rod 10,
10' more or less represents the setting element for the regulation or
tracking of the primary mirror arrangement, whereby the electric motor
11, 11', 11'' is also part of this setting element. The triggering or
regulation of the linear motors 11, 11', 11'' is implemented by means of
a common regulator 12.

[0034]FIGS. 4 and 5 show a roller bearing block 15 in which a tracking
shaft 17 is guided. The tracking shaft 17 connects the primary mirrors 6,
6' of a primary mirror group and ensures parallel rotation of all the
mirrors of this group as a result of tracking initiated by the movement
of a connecting rod 10, 10'. The roller bearing block 15 surrounds the
tracking shaft 17, whereby the shaft is mounted on roller elements 16,
16', 16'' in the roller bearing block 15. These roller elements 16, 16',
16'' are essentially cylindrical, but have concave mantle surfaces on
which the tracking shaft 17 is supported. As a result of this barrel-like
shaping, it is possible to position the tracking shaft 17 at a slant as
shown in FIG. 5, whereby the roller bearing block 15 remains in its
perpendicular position. This allows laying the tracking shaft 17 along
slanted surfaces, for example on hills or on uneven terrain. In this
connection, one must, of course, ensure that the receiver 1 is not
covered up relative to the primary mirrors 6, 6' in question.

[0035]Thus a Fresnel solar collector arrangement is described above, which
is temperature-compensated, to a great extent, in that materials having
the same thermal expansion coefficients are used for the supporting rails
5 of the mirror supporting framework 4 and the receiver masts 2 and that
furthermore, the receiver mast and the mirror supporting framework 4 are
mounted in constraint-free manner. Beyond that, tracking of the primary
mirrors 6, 6' to follow the position of the sun is significantly
simplified by means of mechanical coupling of the primary mirrors 6, 6'.